Crustal Evolution in South India: Constraints from Nd Isotopes

Abstract

Nd-isotope analyses from a range of lithologies, including charnockites, metasediments, and granitoids that comprise the high-grade terrain of South India define contrasting age provinces. North of the Palghat-Cauvery shear zone, the Karnataka Craton and the granulite blocks of Nilgiri and Madras are characterized by model Nd ages from 3.4 to 2.4 Ga with significant crustal growth during the Late Archean. South of the Palghat-Cauvery shear zone a Pan-African granulite-facies event resulted in Sm/Nd fractionation, as observed in several incipient charnockite localities. Model Nd age calculations that allow for Sm/Nd fractionation at 550 Ma yield ages in the range of 2.9 to 1.3 Ga. This range compares closely with published model Nd ages from the high-grade terrain of Sri Lanka. Mid-Proterozoic crustal growth (~1.1 Ga) observed in the Vijayan Complex of Sri Lanka may have provided source material for metasediments characterized by young model Nd ages in southern Peninsular India. The Palghat-Cauvery shear zone therefore represents a boundary between southern terranes that are characterized by crustal reworking and high-grade metamorphism during the Pan-African tectonothermal event and an Archean craton to the north where no Pan-African overprint has been detected. Charnockite formation is identified both with the period of Late Archean crustal growth and with the period of Pan-African metamorphism, as can be observed not only in southern India, but also in the proximal Gondwanan fragments of East Antarctica and Madagascar.